1. Field of the Invention
The present invention relates to an ion trap mass spectrometry and an ion trap mass spectrometer.
2. Description of the Prior Art
Fundamental configuration and operation of an ion trap are disclosed in U.S. Pat. No. 2,939,952 by Paul et al.
In addition, mass spectrometers using ion traps are disclosed in Japanese Patent Laid-Open No. 59-134546, Japanese Patent Laid-Open No. 62-37861, Japanese Patent Laid-Open No. 7-146283, Japanese Patent Laid-Open No. 10-294078, and U.S. Pat. No. 5,734,162.
As disclosed in the above-mentioned publications, an ion trap mass spectrometer has a ring electrode and a pair of end cap electrodes, which form an ion trap region to trap ions.
Fundamental operation of an ion trap mass spectrometer with an electron impact (EI) ion source includes an ionization step in which a sample in an ion trap region is ionized by allowing it to collide with electrons, and resulting ions are accumulated in the ion trap region, and a mass analyzing step in which the accumulated ions are consecutively ejected from the ion trap region by scanning of radio frequency (Rf) voltage applied to the above-mentioned electrodes, and the ejected ions are detected by a detector. Thus, fundamental operation of mass analyzing is to go through each of the steps with the lapse of time.
In the mass analyzing step described above, there should not be new ionization, external ion injection, or the like in the ion trap region. If ionization or ion injection in the ion trap region occurs during mass analyzing, ions are ejected from the ion trap region to the outside regardless of their masses during main high frequency voltage scanning for mass analyzing. The ejected ions are detected by a detector. This results in random noise that appears on a mass spectrum.
For example, suppose that ions having a mass number of 200 and a mass number of 250 are generated in the ion trap region at the moment when a high frequency applied to the ring electrode is being scanned and thereby ions having a mass number of 300 are to be ejected. The ions having a mass number of 200 and a mass number of 250 immediately become unstable in the ion trap region due to a quadrupole Rf field in the ion trap region. The ions are immediately ejected from the ion trap region to the outside, resulting in noise before and after the mass number of 300 on a mass spectrum.
Thus, in an ion trap mass spectrometer, the ionization step and the mass analyzing step are strictly separated by controlling electrons by means of an electron gate so that occurrence of noise can be prevented.
In actuality, however, even with an ion trap mass spectrometer using the above-mentioned electron gate, spike noise occurs occasionally on a mass spectrum. FIG. 5B shows a mass spectrum when noise has occurred. In the figure, m3 denotes a molecular ion resulting directly from ionization of a sample molecule, while m1 and m2 denote fragment ions resulting from cleavage of the molecular ion. A spectrum to appear should include only m1 to m3, as shown in FIG. 5A; however, in actuality, many mass peaks other than m1, m2, and m3 appear, and thus a mass spectrum as shown in FIG. 5B is obtained. In the figure, noise is denoted by a symbol n written on top of a mass peak. Of course, n, m1, m2, and the like are not written on an obtained mass spectrum. As a result, it is impossible for the observer to distinguish between signals and noise. Some of the noise peaks result from ionization of background components other than sample components. These noise peaks are reproducible, and therefore distinguishable. In the case of high-sensitivity measurement in which very small quantities of components are measured, however, random noise appears in addition to the above noises. Since the noise is a random noise occurring irrespective of mass number, it is quite impossible to identify ions that cause the noise. Furthermore, the noise could make it impossible to perform high-sensitivity quantitative analysis. The noise may ruin the characteristic of an ion trap mass spectrometer of being highly sensitive.
An object of the present invention is to solve such problems and allow high-sensitivity measurement of an ion trap mass spectrometer.
Several factors can be considered as the causes of random noise; however, it has been found as a result of experiments by the inventor that the following are the two main causes of random noise.
(First Cause) Ions are injected into an ion trap region in the mass analyzing step.
As described above, in the mass analyzing step, an electron gate is closed (application of a negative voltage) so that electrons will not enter an ion trap region. However, in order to stabilize emitted electrons, a filament is supplied with a current from a filament power supply at all times. Therefore, in the vicinity of the tip of the filament, there exist in large numbers electrons emitted from the filament as well as electrons and other particles reflected from a grid electrode and the like. On the other hand, pressure around the periphery of the filament represents 10xe2x88x923 Pa to 10xe2x88x924 Pa, and thus many residual gases are present there. When the residual gases and electrons in the vicinity of the filament collide with each other, gaseous molecules are ionized to form positive ions. The positive ions are accelerated by a negative voltage applied to the electron gate electrode, and then enter the ion trap region. The ions are immediately ejected from the ion trap region and then detected by a detector, thereby resulting in random noise.
(Second Cause) Electrons, photons, and ions emitted from the filament directly enter the detector.
As a detector of a mass spectrometer, a detecting system using a secondary electron multiplier or a photomultiplier in which ions are converted into electrons to emit light by means of a scintillator is employed. In addition, not all the electrons and photons emitted from the filament enter the ion trap region; some are reflected in a diffused manner by a wall surface or the like inside the vacuum vessel that houses the mass spectrometer. Such electrons and photons directly enter the detector, thereby causing noise. Furthermore, accelerated electrons ionize residual gas molecules in the vacuum vessel on the way to the detector. When the resulting ions directly enter the detector, it also results in noise.
The present invention has been made to solve such problems. Specifically, an electron gate electrode situated between a filament and an end cap electrode is divided into two pieces, whereby voltages applied to the respective pieces are controlled independently of each other during ionization and during mass analyzing. This prevents undesired ions and electrons from being injected into an ion trap region during mass analyzing.
In addition, according to the present invention, a plurality of cylindrical or plate electrodes for shielding electrons, ions, and photons are disposed between the filament and a detector. This makes it possible to prevent ions, electrons, and other particles scattered in a vacuum vessel from directly entering the detector.